Systems have already developed for keying time compressed audio into unused lines in the vertical interval of a video signal. However, most of the known systems have given rise to poor quality audio reception, particularly when transmitting multi-channel audio, because of difficulties that have been experienced in reconstituting the audio from the discontinuous signals. The present invention has largely overcome these problems with the development of its so-called "VIMCAS" system (disclosed in Australian Patent No. 568443) which accurately synchronises the encoding/decoding of the audio, using the synchronising information and colourburst signal in the video, and which also provides for analogue redundancy in the encoded audio.
However, the prior art systems suffer from the problem of audio distortion caused by the presence of what are herein referred to as "distortion signals" in the audio. These distortion signals may arise from apparatus faults, from reflections in a transmission line, from interference in free space radiation or as a consequence of system switching, to name but a few possible sources.